This invention relates to a composition and a method of improving the properties of power transmitting fluids, particularly to obtaining automatic transmission fluids of improved viscosity control.
Automatic transmissions continue to become more sophisticated in design as vehicle technology advances. These design changes result from the need to improve vehicle operability, reliability, and fuel economy. Vehicle manufacturers worldwide are increasing vehicle warrantee periods and service intervals on the vehicles. This means that the automatic transmission, and the automatic transmission fluid (ATF), must be designed to operate reliably without maintenance for longer periods of time. In the case of the fluid, this means longer drain intervals. To improve vehicle operability, especially at low temperature, manufacturers have imposed strict requirements for fluid viscosity at -40.degree. C. To cope with longer drain intervals and more severe operating conditions, manufacturers have increased the requirements for oxidation resistance of the ATF, and increased the amount of wear protection that the fluid must provide for the transmission. To improve the fuel economy of the vehicle and reduce energy loss in the torque converter, manufacturers employ sliding torque converter clutches, which require very precise control of fluid frictional properties. One common element in the quest for better reliability, longer service life, and better transmission control is the viscometric properties of the fluid.
It is well known that lowering the viscosity of an ATF at low temperatures (e.g., -40.degree. C.) will result in improved operability of the transmission at low ambient temperatures, that increasing the amount of antiwear additives in the ATF will result in more wear protection, and that better friction control can be obtained by judicious choice of friction modifiers. However, we have now found that by proper selection of viscosity modifier molecular weight, the low temperature operability, service life, and friction control of the ATF, can be improved simultaneously.
Correct choice of the viscosity modifier molecular weight allows the fluid to meet the high temperature viscosity requirements imposed by the manufacturer, while also allowing the fluid to meet rigorous low temperature viscosity limits. High temperature viscosity is also known to control wear in hydrodynamic and elastohydrodynamic wear regimes. High initial viscosity, at high temperatures (e.g., 100.degree. C. and 150.degree. C.), at both low (i.e., 1 to 200 sec..sup.-1) and high shear rates (1.times.10.sup.6 sec..sup.-1) helps to control this wear. Equally important is the fluid's ability to maintain this high level of viscosity under both high and low shear rates, even after use. The high initial viscosity at high temperatures and low shear rates is important to transmission operability as well. High viscosity at high temperature and low shear rate controls fluid leakage at high pressures. This is not leakage from the transmission itself, but leakage at high pressures (e.g., 830 kPa (120 psi)) around seals and valves in the transmission control system. No matter how sophisticated the electronic control of the transmission, if the fluid is leaking under pressure in the valve body, the transmission will not function properly. This is particularly important in transmissions using sliding torque converter clutches since control of these devices is accomplished via minute fluctuations in clutch actuating pressure.
We have found that by careful selection of the molecular weight of the viscosity modifier, the aforementioned properties of the ATF can be improved simultaneously. If the molecular weight of the viscosity modifier is too low, too much will be needed to produce the required viscosity at high temperatures. This is not only uneconomical, but will eventually cause elevation of the viscosity at low temperature making it difficult or impossible to meet lower -40.degree. C. Brookfield viscosities. If the molecular weight of the viscosity modifier is too high, it will degrade by both mechanical shear and oxidation during service such that the high temperature viscosity contributed by the polymer will be lost, making the transmission vulnerable to wear and internal leakage. However, adding sufficient high molecular weight polymer to give the required "used oil viscosity" causes elevation of the low temperature Brookfield viscosity of the fluid, possibly exceeding the specified maximum viscosity.
Since fluids exhibiting the characteristics of this invention must have exceedingly good low temperature fluidity (i.e., Brookfield viscosity .ltoreq.15,000 centipoise (cP) at -40.degree. C.), it is necessary to use a lubricating oil that contains a synthetic base oil in an amount sufficient to obtain the desired Brookfield viscosity.
ATF's provide very precise frictional characteristics to the transmissions that they are used in. To meet this requirement, they must contain friction modifiers.